The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
One of the obstacles of long-distance quantum communication is that the degree of entanglement generated between two distant quantum systems coupled by photonic channels decreases exponentially with the length of the connecting channel due to optical absorption and noise in the channel. Exponential decay of entanglement as a function of channel length requires an exponentially increasing number of partially entangled states to obtain one highly entangled state. Similar problems exist in the proposals of scalable quantum computing. As an example, trapped ions constitute one of the most promising systems for implementing a quantum computer. It appears unlikely, however, that this system can be scaled by simply adding ions to a single trap due to the growing complexity of the vibrational mode spectrum and the inefficiency of laser cooling of the collective motion of a large ion crystal to near the ground state.
To overcome the problems of scalability of quantum computing and long-distance communication, the idea of using quantum repeaters has been proposed. A key aspect of quantum communication involves using quantum repeaters to divide a long communication channel into a sufficiently large number of short segments of length L0 that are less than the attenuation length Lat. Information is then transferred between adjacent segments via entanglement swapping.
Unfortunately, the proposals for quantum repeaters are ostensibly theoretical and do not address the practical aspects of implementing quantum repeaters in a manner suitable for a commercially viable quantum computer or long-distance communication system.